The present invention relates to an optical recording medium such as a writable disc and readable disc, and more particularly to a recording medium which has a recording film containing coloring matter of phthalocyanine.
In a recording film of a rewritable optical recording medium, it is well known to use organic coloring matter such as cyanine or phthalocyanine.
As a method for writing information on the recording medium, a laser beam is focused on the recording film at a small area and converted into thermal energy so that the characteristic of the recording film is changed to form a pit. In order to properly change the characteristic of the recording film, two sheets of the recording mediums comprising a substrate and a record film coated on the substrate is prepared. The recording films are disposed opposite to each other, interposing air therebetween, so called air sandwich construction.
A transparent substrate is irradiated with a laser beam for writing information and the pit is formed on the recording film. A laser beam for reading the recorded information has a rather weak power compared with the writing laser beam. The contrast between the pit and the area other than the pit is detected as an electric signal.
Besides the above mentioned recording medium, a recording medium on which information has been already recorded is provided. It is a read only memory (ROM) type recording medium which is widely used in the sound recording field and information processing field. In such a recording medium, it is not necessary to provide the writable recording film and prepits corresponding data to be reproduced are previously formed on a plastic substrate by press. A metallic reflection film made of Au, Ag, Cu, or Al is coated on the substrate and a protection film is covered on the reflection film. The most typical example thereof is a compact disc (CD). The format of writing and reading signal of the CD is standardized and the reproducing device of the CD is widely used as a CD player.
Since the rewritable recording medium uses a laser beam and is in the form of a disc, it is the same as the CD. Therefore, it has developed to adapt the recording medium to be played by the CD player.
The recording medium comprises a light-transmissible substrate, a recording film made of organic coloring matter and coated on the substrate, a light reflection film coated the recording film, and a protection film covered on the reflection film.
When using, the recording medium is reproduced by a CD player mounted on a car. Therefore, it is desirable that the recording medium has high resistances to light and environmental conditions. For such a use, it has been found that the phthalocyanine coloring matter is more effective for the recording film than the cyanine coloring matter.
In the recording medium having the recording film of phthalocyanine, a light reflection film made of Au is coated on the recording film. If the thickness of the reflection film is large, recording sensitivity is reduced and a hump is generated in a waveform of a reproduced signal.